۱- Introduction

۲- Experimental programme

۳- Numerical modelling

۴- Proposed design method

۵- Conclusions

References

ABSTRACT

The objective of this work is to develop design guidance for existing hollow steel columns that are retrofitted by infilling concrete into the tubes. The primary challenge is the unknown effect of the existing preload on the steel columns prior to the concrete infill. Composite performance and buckling behaviour of circular hollow section (CHS) steel columns strengthened by infilling concrete under preload was experimentally and numerically investigated in this study. A total of 34 CHS steel columns were tested under pin-ended boundary conditions, and the overall buckling failure modes and corresponding ultimate buckling resistances were recorded. Prior to the member testing, material properties of the steel columns and the infilled concrete were attained. By means of the finite element (FE) software package ABAQUS, elaborate FE models for the CHS columns strengthened by infilling concrete were developed and validated against the obtained test results, which were further verified with other available test data. Using the validated FE models, systematic parametric studies were conducted to examine the influences of the major factors affecting the ultimate capacities of the CHS columns strengthened by infilling concrete, including preload ratios, steel and concrete strengths, steel ratios, initial global imperfections, eccentricity ratios and column slenderness ratios. The obtained test and numerical results were therefore utilised to develop design criteria for predicting the overall buckling resistance of CHS steel columns strengthened by infilling concrete. In view of the difficulty of determining the key parameters in practice and the uncertainty of the strengthening process, a new simplified design coefficient was proposed, taking into account the influence of the preload. It has been demonstrated that accurate and reasonable strength predictions can be provided by the proposed design method.

Introduction

Strengthening existing steel structures is often imperative due to additional imposed loads or alterations of primary function [1]. Methods used to reinforce steel structures include welding cover plates [2–۴], bonding steel plates or fibre reinforced polymer materials [5–۷], external post-tensioning using tendons [8–۱۰]. For steel tubular columns, infilling concrete into the tube columns is another feasible retrofit technique that has little effect on both the appearance and the net area. The infilled concrete benefits from the confining effect provided by the steel tube and may also delay the occurrence of local buckling [11–۱۴]. As a result, the load-carrying capacity of an existing steel tubular column can be significantly enhanced. It is noted that the infilling concrete will introduce additional self-weight to the structural system, yet this can be considered in the retrofitting process, with additional design checks to be conducted. Generally, it would be essential to unload the columns prior to strengthening. However, it is inconvenient and often infeasible to unload the column before strengthening, therefore the strengthening process is carried out while the member is subjected to self-weight and external actions [15–۱۷]. The composite strength and flexural stiffness is sensitive to this existing load; therefore, in this paper, the overall buckling behaviour of circular hollow section (CHS) steel columns strengthened by infilling concrete under preload is investigated.